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A controversial new study has found not having enough salt in your diet may actually be bad for you and suggests campaigns telling people to cut down on salt may only be worth it in countries with very high sodium consumption.

The World Health Organisation recommends capping salt consumption at 5g per day — about a teaspoon — because of the risks associated with increased blood pressure and stroke, reports news.com.au.

But this target is not known to have been achieved anywhere in the world, note the authors of the study published in The Lancet medical journal.

"We should be far more concerned about targeting communities and countries with high average sodium intake — above 5g (equivalent to 12.5g of salt), such as China — and bringing them down to the moderate range" of 7.5 to 12.5g of salt, said lead author Andre Mente, a professor in the Population Health Research Institute at McMaster University in Canada.

"By adopting heart healthy eating patterns which includes a combination of foods chosen regularly over time, Australians can reduce their salt intake," Ms Mitchell said.

Contrary to this, Professor Mente also said: "Our study adds to growing evidence that, at moderate intake, sodium may have a beneficial role in cardiovascular heath, but a potentially more harmful role when intake is very high or very low".

The human body needs essential nutrients such as sodium and many vitamins, but the ideal amount remains subject to debate.

The study, which stopped short of calling for WHO recommendations to be relaxed, examined urine and blood samples, along with health records, for 95,767 women and men monitored over an eight-year period.

Nearly 3700 of the participants died during that time and 3543 had "major cardiovascular events".

Experts not involved in the study were sharply critical of its methodology, and said its findings should be taken with more than a few grains of salt.

The technique for collecting urine samples is notoriously unreliable, they noted.

And the fact that it was an observational study — as opposed to clinical trials — means that no firm conclusions can be drawn as to cause-and-effect.

Most controversial was the suggestion that low sodium intake may, in fact, provoke heart disease.

"There are no known mechanisms that could explain this observation," said Tom Sanders, professor emeritus of nutrition and dietetics at King's College London.

"Sodium is an essential nutrient but the requirement is very low at about 0.5g (1.25g of salt) per day."

Ageing populations, he added, should still be advised to restrict the addition of salt to food.